If you bought a Kontakt Player library such as Sunset Strings, Realivox Ladies, Realivox Blue, Fingerpick, Realidrums, or Hip Hop Creator, then your serial number is located at the top of your unique download page, accessible from the confirmation e-mail you received upon purchasing your library. If your download includes more than one link, your license will be located under the last one at the bottom of the list, as shown below:
NOTE: If you purchased the cheaper "Full Kontakt" version of any of the RealiVox product series (Ladies, Teresa, Blue), then there is no serial number necessary.
For RealiBanjo, RealiWhistle, Screaming Trumpet, and Acoustic Lite, there is also no serial number necessary.
This means there's no need to "Add Library" inside Kontakt or Native Access. To load, you simply click the "Files" button at the top of the Kontakt interface, or the "Files" tab in the sidebar browser, or using Quickload. And You're all set!